Bob's theory; A 5 gallon tank with 100 psi will fill a 1 gallon tire 20 psi and have 5 gallons of 80 psi air left in it. That is considering no loss of air during the filling process. Most off road tires are bigger than 5 gallons.

Riddle: If you have a 23 gallon air tank filled to 200 PSI air pressure and you fill one 37"X12:50X15 MTR from 10 PSI to 27 PSI how many gallons of air are left in the tank?

Bob's theory; A 5 gallon tank with 100 psi will fill a 1 gallon tire 20 psi and have 5 gallons of 80 psi air left in it. That is considering no loss of air during the filling process. Most off road tires are bigger than 5 gallons.

My calculations put the tank at 146 psi after filling a 1 gallon tire 20 psi. However a 1 gallon tire is tiny. The tire in your example below is over 40 gallons.

Quote:

Originally Posted by Bigbob

Riddle: If you have a 23 gallon air tank filled to 200 PSI air pressure and you fill one 37"X12:50X15 MTR from 10 PSI to 27 PSI how many gallons of air are left in the tank?

Trick question: the tank doesn't change size, so there are still 23 gallons of air. The pressure has changed. If anyone is curious what the pressure would be:

If you assume a perfectly cylindrical tire on a 12.5" wide rim (probably not the case), then your tire is 48.6 gallons. Starting with a 23gal tank at 200 psi and a 48.6 gal tire at 10 psi, and ending with a 48.6 gal tire at 27 psi, the tank (still at 23 gallons) would have a pressure of 164 psi.

Now wait a minute... I used to do this all the time on my 2004 Rubicon.

The stock MTRs are rated to maximum load at something like 85 psi. This maximum psi is right on the side of the tire. I would fill the spare tire to 80 or 85 psi before a wheeling trip and then use this air to refill the other 4 tires. It was enough air to go from around 15 psi back to about 30 psi. Worked like a champ.

However, I have since purchased a small portable 12 volt air compressor that works pretty good. They are sold at stores like Pep Boys and Autozone for about $50 bucks. It much more powerful than the small ones everyplace sells for around $25 bucks. It will air up a stock MTR from 15 psi to 33 psi in about 3 to 4 minutes. Not a bad time for the price.

I was thinking of doing this but I never tried it. If it worked so well, why did you buy a portable compressor? In your opinion, which works better, the spare tire or the compressor?

My calculations put the tank at 146 psi after filling a 1 gallon tire 20 psi. However a 1 gallon tire is tiny. The tire in your example below is over 40 gallons.

Trick question: the tank doesn't change size, so there are still 23 gallons of air. The pressure has changed. If anyone is curious what the pressure would be:

If you assume a perfectly cylindrical tire on a 12.5" wide rim (probably not the case), then your tire is 48.6 gallons. Starting with a 23gal tank at 200 psi and a 48.6 gal tire at 10 psi, and ending with a 48.6 gal tire at 27 psi, the tank (still at 23 gallons) would have a pressure of 164 psi.

LOL You are an egghead aren't ya! I just wanted to catch someone trying to figure out how many gallons of air is left in the tank. And try to get across that hauling around a 5 gallon tank with 150PSI of air in it is sort of senseless.

Only problem is that it's huge and blocks a to of airflow to the radiator. Might not be a big deal in the north, but for us down here that see 100+ that ain't gunna work.

it also violates the KISS principle. if something goes wrong with everything that is involved in the unit, you are out of a) recovery and b) air.
i'd rather eat up space with trail gear than be stuck out there...